This invention generally relates to apparatus for classifying particles; and, in particular, to apparatus for separating particles smaller than a given size--referred to as the particle classification or cut-point size--from a mixture of particles.
More specifically, the present invention relates to a particle classifier comprising a vertically disposed classification chamber with a cylindrical top part and a funnel-shaped bottom part, and a separator mounted in said top part and having the form of a rotating wheel. The rotating wheel has a multitude of blades arranged in the form of a blade ring, with each blade extending parallel to the axis of rotation of the wheel. Material to be classified is conveyed, either together with air entering through a lower opening in the funnel-shaped bottom part or directly, into the classification chamber. Coarse material is carried away through the lower opening of the funnel-shaped bottom part; and fine material, together with the air from the rotating wheel, is drawn off through an outlet exiting upward from the classifier.
A classifier of the type described above wherein the material to be classified is conveyed to the classification chamber while dispersed in air, is described, for example, in British Pat. No. 927,876. Another classifier of the same general kind but with direct conveyance of the material being classified to the separation chamber is shown in German Pat. No. 17 57 582, which corresponds to U.S. Pat. No. 3,384,238. The particle classification size achievable with such prior art classifiers is essentially a function of the diameter and circumferential velocity of the separating wheel. Thus, with a given size of wheel, the higher the circumferential velocity of the wheel, the smaller is the particle classification size. With an increase of the wheel velocity, however, there is also a rapid increase in wear and energy consumption so that an economical operation can be realized only if the particle classification size is kept above a certain size.
On the other hand, when the circumferential velocity of the wheel and the radial velocity of the separating air at its entry into the wheel at the wheel circumference are kept constant, the particle classification size becomes greater with an increasing wheel diameter, i.e. it is moved into a larger size range. If therefore, a large separating wheel is to have the same particle classification size as a small one, then the circumferential velocity of the larger wheel must be greater than the circumferential velocity of the smaller wheel. This means, however, that in addition to increased wear and energy consumption, stability and bearing problems also have to be expected. As the diameter of the separator wheel increases, the capacity of the wheel also increases.